This invention relates to vertical cavity surface emitting lasers capable of emitting long-wavelength light and particularly to electrically pumped long-wavelength vertical cavity surface emitting lasers and to methods of fabrication.
Vertical cavity surface emitting lasers (VCSELs) include first and second distributed Bragg reflectors (DBRs) formed on opposite sides of an active area. The VCSEL can be driven or pumped electrically by forcing current through the active area or optically by supplying light of a desired frequency to the active area. Typically, DBRs or mirror stacks are formed of a material system generally consisting of two materials having different indices of refraction and being easily lattice matched to the other portions of the VCSEL. In conventional VCSELs, conventional material systems perform adequately.
However, new products are being developed requiring VCSELs which emit light having long-wavelengths. VCSELs emitting light having long-wavelengths are of great interest in the optical telecommunications industry. This long-wavelength light can be generated by using a VCSEL having an InP based active region. When an InP based active region is used, however, the DBRs or mirror stacks lattice matched to the supporting substrate and the active region do not provide enough reflectivity for the VCSELs to operate because of the insignificant difference in the refractive indices between the two DBR constituents.
Dielectric mirror stacks can be used for VCSEL applications, but they suffer from poor thermal conductivity. Since the performance of these long-wavelength materials is very sensitive to temperature, the thermal conductivity of the DBRs is very important. At least one of the DBRs must have good thermal conductivity to dissipate the heat generated by the laser.
A metamorphically grown DBR has good thermal conductivity and can be used as a heat conducting DBR, as described in United States of America Patent Application entitled xe2x80x9cMethod of Fabricating Long-Wavelength VCSEL and Apparatusxe2x80x9d, filed on Aug. 21, 2000, Ser. No. 09/642,359, and incorporated herein by reference. However, it is very difficult to make good electrical contact through the metamorphic DBR, since the interface between the highly defected metamorphic DBR and the active structure results in a high series resistance. Further, any attempts to provide electrical contacts within the structure, instead of at the surfaces, results in current confinement problems that can seriously affect the operation and efficiency of the VCSEL.
Accordingly it is highly desirable to provide electrically pumped long-wavelength VCSELs with good thermal conductivity and methods of fabrication.
It is an object of the present invention to provide new and improved electrically pumped long-wavelength vertical cavity surface emitting lasers.
It is another object of the present invention to provide new and improved electrically pumped long-wavelength vertical cavity surface emitting lasers in which materials with good thermal conductivity and large differences in refractive indices are used.
It is still another object of the present invention to provide new and improved methods of fabricating electrically pumped long-wavelength vertical cavity surface emitting lasers.
It is a further object of the present invention to provide new and improved methods of fabricating electrically pumped long-wavelength vertical cavity surface emitting lasers incorporating materials with good thermal conductivity and large differences in refractive indices.
The above objects and others are realized in an electrically pumped long-wavelength VCSEL which includes a long wavelength active region having electrical contacts on opposed sides thereof. A layer of material is included in the active region having an electrically conductive portion defining a lasing aperture and current confinement volume with the conductive portion being limited by an electrically insulating portion. In a preferred embodiment, the layer of material contains aluminum, e.g., InAlAs, which is laterally oxidized from the outer edges to form the electrically insulating portion surrounding the electrically conductive portion. Windows are formed in the electrical contacts in alignment with the lasing aperture and mirror stacks are positioned on the long wavelength active region in each of the windows. At least one of the mirror stacks includes a metamorphic distributed Bragg reflector for heat conduction. In the preferred embodiment, the active region is an InP based material epitaxially grown on an InP based substrate.
The above objects and others are further realized in a method of fabricating an electrically pumped long-wavelength VCSEL including a step of forming a long wavelength active region with a first electrical contact on one major side and a second electrical contact on the opposite major side. The long wavelength active region includes at least one layer of material having an electrically conductive portion defining a lasing aperture and current confinement volume, which is limited by an electrically insulating portion. In a preferred embodiment, the layer of material includes an easily oxidizable metal, e.g. aluminum, which is laterally oxidized to produce a centrally located electrically conductive portion defining a lasing aperture and current confinement volume surrounded by the electrically insulating portion. A window is formed in the second electrical contact in alignment with the lasing aperture and a first distributed Bragg reflector mirror stack is metamorphically deposited on the long wavelength active region in the first window. A window is formed in the first electrical contact in alignment with the lasing aperture and a second mirror stack is deposited on the long wavelength active region in the second window. Additional metal is deposited on the first and second metal contacts for support and good electrical conduction.